1. Field of the Invention
The present invention relates to a resonator, a frequency filter, a duplexer, and an electronic device that utilize bulk acoustic waves in a piezoelectric layer, and to a method of manufacturing a resonator.
2. Description of Related Art
When an electric field is applied to a piezoelectric element, elastic waves can be generated. Elastic waves include surface acoustic waves (SAW) that propagate along a surface of the piezoelectric element, bulk acoustic waves (BAW) that propagate inside the piezoelectric element, and the like, and the bulk acoustic waves include Lamb waves that propagate as longitudinal waves and transverse waves.
A resonator is an element that utilizes such elastic waves in the piezoelectric element, and is provided with a piezoelectric layer formed on a substrate and an electrode such as a comb-shaped electrode (IDT: inter-digital transducer) formed on the piezoelectric layer. When the electrode is applied with an electric signal, elastic waves are generated in the piezoelectric layer. When elastic waves in the piezoelectric layer reach the electrode, an electric signal can be obtained. The frequency of elastic waves that resonate with the electrode can be controlled by the thickness of the piezoelectric layer and a pitch of the comb-shaped electrode.
Due to such characteristics, a resonator can be used as a frequency filter that allows signals having a specific frequency to pass through, and is widely used in a duplexer that divides reception signals and transmission signals in a communication apparatus.
Among elastic waves, the bulk acoustic waves propagate inside the piezoelectric layer, and therefore, a free vibration surface that allows the piezoelectric layer to vibrate needs to be provided on the side opposite to the electrode, that is, at the boundary between the piezoelectric layer and the substrate. For example, in the “Lamb wave high-frequency device” disclosed in Patent Document 1, the reinforcing substrate has a recess formed therein, thereby providing a free vibration surface for the piezoelectric layer.